API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

5-34 API RECOMMENDED PRACTICE 581
AU
tgt
ηtgt
% = (5.75)
η
mod
tube
The AU tgt
% is used with Table 4.5 to determine the level of inspection required to achieve target P at
f , tgt
the Plan Date. The inspection plan is defined by using the target inspection date from Equation (5.71) and
the recommended inspection from Equation (5.75).
Calculate Characteristic Life at Plan Date
The recommended inspection uncertainty is used calculate the characteristic life at the plan date after
inspection using Equation (5.76).
η
Where
η
⎛1 − AU % ⎞
w/
insp
insp
=
mod
⋅⎜
⎜
1 − AU
noinsp
% ⎟
⎝
η
mod
is defined in Equation (5.62).
Calculation of Risk
⎠
(5.76)
tube
The POF at the plan date, P , with inspection is calculated with Equation (5.55) using
f , w/
insp
t for time at
plan
the plan date, ηinsp
from Equation (5.76) and the original β value.
Calculation of Risk
tube
tube
The Risk at the plan date is calculated using Equation (5.70) using P and
f , w/
insp C .
f , plan
Bundle Inspect/Replacement Decisions using Cost Benefit Analysis
Weibull parameters are used to predict the optimal replacement frequency for a bundle and determine
whether it makes economic sense to inspect or replace a bundle at an upcoming shutdown.
Decision to Inspect or Replace at Upcoming Shutdown
Risk reduction cost benefit is calculated from mitigating actions including various levels of inspection or
bundle replacement. The cost benefit calculation includes the cost of the mitigating action to inspect or
replace). An optional rate of return, ROR (or hurdle cost), is added to the cost of a bundle replacement to
prevent excessive bundle replacement for borderline risk determinations that require action. The decision to
perform a risk mitigating bundle inspection or bundle replacement at an upcoming turnaround is determined
by comparing the incremental risk ($) associated with deferring the inspection or replacement to the cost
associated with the performing the inspection or replacement.
t 2
Expected incremental risk, EIR , associated with deferring the inspection or replacement of a bundle to a
t1
later date is calculated using Equation (5.77).
EIR
⎛
⎡1−
P
tube
t 2 tube
f
t1
= C
f
⋅ 1−
⎜
⎢ tube
1−
Pf
⎝
⎢⎣
( t2
)
( t )
1
⎤⎞
⎥
⎥⎟
⎦⎠
(5.77)
Wheret 1
is the service duration of the bundle at the next shutdown (Turnaround Date 1) and t 2
is the service
duration of the bundle at the subsequent shutdown (Turnaround Date 2).
If the cost to inspect or replace is greater than the expected incremental risk using Equation (5.78) and
Equation (5.79), that action is recommended.
t 2
( insp
+
maint ) ⋅ ( 1 + ) <
t1
2
( ) t
insp maint t1
If Cost Cost ROR EIR then inspect
If Cost + Cost + Hurdle Cost < EIR then inspect
(5.78)